A long lubrication life is required for a grease which lubricates a portion such as rolling bearings for automobile electrical equipment or auxiliaries, which is heated to extremely high temperatures and subjected to extremely high speed movements. In order to provide a long lubrication life, the properties of the grease to easily flow into the lubrication unit, as well as heat-resistant property, are required. Such an inflow property of the grease is particularly strongly required when outer race rotation type bearings are used for the bearings, since it is more difficult for the grease to flow into the lubrication unit, for example due to the action of centrifugal force in the outer race rotation type bearings. On the other hand, when the inflow property is too high, the grease is soft and leaks from the bearings, and therefore the lubrication life becomes short. Thus, a grease which has a good balance between heat-resistant property and inflow property is demanded.
Examples of outer race rotating bearings for automobile electrical equipment or auxiliaries include bearings used for electromagnetic clutch, intermediate pulley, idler pulley, tension pulley and the like used for automobile air conditioners. On the other hand, examples of inner race rotating bearings include bearings used for alternators and the like.
The grease which has a long life when used for outer ring rotation type bearings may be used not only for the outer ring type bearings but also for inner race rotation type bearings since, as described above, the grease has a good balance between heat-resistant property and inflow property.
Diurea greases which employ mainly a synthetic hydrocarbon oil or phenyl ether synthetic oil as a base oil are used as the grease used for rolling bearings for automobile electrical equipment or auxiliaries. In many cases, an aromatic diurea grease is used among diurea greases since it has an excellent high-temperature resistance. However, the lives of this sort of greases are not nearly satisfactory when used for outer race rotation type bearing because of insufficient inflow property of the greases into the lubrication unit.
In the meantime, as an example of improving the inflow property by a thickener, Patent Document 1 describes a grease composition in which the diurea compound represented by the following general formula is employed as a thickener in a base oil containing an ether synthetic oil in an amount more than 50% by mass of the base oil:R1—NHCONH—R2—NHCONH—R3,wherein R2 represents C6-15 divalent aromatic hydrocarbon group, R1 and R3, the same or different, represent cyclohexyl, C7-12 cyclohexyl derivative groups or C8-20 alkyl; and the ratio of the number of moles of cyclohexyl and derivative groups thereof to the total number of moles of cyclohexyl, C7-12 cyclohexyl derivative groups and C8-20 alkyl is 50 to 100%
However, this grease composition lacks in compatibility between the base oil and the thickener, lacks in inflow property, and the life of the bearing is not satisfactory.
Patent Document 1 JP 2979274B